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Valve's official stance is that they do not intend on providing a definitive Half-Life / Portal series timeline as they fear the possibility of contradicting themselves would limit their creativity in future games. Since the story is still in the process of being written, there is always a chance that facts will be altered at a later date.[1] This timeline has been built by gathering every known date with the known events and chronological facts organized around them. Several retcons have been made since the release of the first game; they have been applied here with the altered dates denoted.

Aperture Science is rated #2 among the Top 100 Applied Science Companies by the Mechanical Engineering World Journal. It is implicit that the number one spot was achieved by Black Mesa, but nothing firmly concludes this, based on the intense rivalry between the two companies.

Aperture Science constructs the Conversion Gel Testing Facility's enrichment center, and now performs tests on its own employees. By now, Cave Johnson has spent $70 million on moon rocks to grind up and turn into Conversion Gel, and as a result of exposure has himself become gravely ill and addicted to pain medication.

Aperture engineers complete the Heimlich Counter-Maneuver and Take-A-Wish Foundation initiatives. The company announces products related to the research in a televised ceremony. These products become immediately wildly unpopular. After a string of very public choking and despondent sick child disasters, senior company officials are summoned before a Senate investigative committee. During these proceedings, an engineer mentions that some progress has been made on Tier 3, the "man-sized ad hoc quantum tunnel through physical space with possible applications as a shower curtain." The committee is quickly permanently recessed, and Aperture is granted an open-ended contract to secretly continue research on the 'Portal' and Heimlich Counter-Maneuver projects.[5]

The Conversion Gel Testing Facility is updated with an additional wall.

Word reaches Aperture Science management that Black Mesa is working on a similar portal technology. In response to this news, Aperture Science begins developing GLaDOS, although the name was already in use in 1982.[5]

While visiting the University of Innsbruck, Gordon Freeman observes a series of seminal teleportation experiments conducted by the Institute for Experimental Physics. Practical applications for teleportation become his obsession.[9]

After a decade spent bringing the Disk Operating System parts of GLaDOS to a state of more or less basic functionality, work begins on the Genetic Lifeform component.[5] During that time, the Aperture Science Red Phone plan is implemented in case GLaDOS appears to become sentient and godlike, requiring an employee to sit by a red phone on a desk in GLaDOS chamber's entrance hall.[10]

This is the "year" during which the Black Mesa Incident occurs. Deliberately kept vague by the Half-Life team,[15] it could be any date from 2000 to 2009.

Deduction points at "May 16, 200-" for the precise date of the incident. What follows are the different facts and clues leading to that choice:

The year "200-", first appearing in the Half-Life instruction manual, is given in every official source from the Half-Life story arc era.

On March 24, 2010, an updated and expanded Aperture Science timeline originally given on ApertureScience.com in 2006 and submitted by Erik Wolpaw was published on Game Informer. Several dates were changed, and the last paragraph was expanded with facts suggesting that the Black Mesa Incident had occurred in 1998, the same date as that of Half-Life's release, instead of the original "200-". However Marc Laidlaw dismissed "1998" as the date for the Black Mesa Incident, as the date "200-" given when the first Half-Life was released is the only correct one.[15] For convenience and consistency, that "1998" will always be replaced by "200-" on the wiki, even with the dates pertaining to Aperture Science.

From Shephard's diary entries: "Another typically hellish day at base camp... I'll be glad when this is over and I can get assigned a mission. There has been this really weird civie spotted at the base. Rumor is he's from some government branch looking to recruit; others say he's with some secret research group. I would jump at the chance to join. It would be cool just for the change and the adventure."[16]

From Shephard's diary entries: "I finally saw the government guy today. I am not sure he is a g-man, but he was wearing a really uptight suit and carrying a briefcase. He looked more like a lawyer or insurance agent to me. I did notice him checking me out. Several times throughout the day I spotted him just watching me during training. I wonder what he's up to..."[16]

From Shephard's diary entries: "For weeks our drills have been the same crap day after day. Today we assemble for the morning run and our drill instructor tells us we have one week to become experts at indoor strategic combat. We will be spending every day this week at the combat simulation facility. As far as I know this a specialized training not taught in boot camp. What I want to know is if this is to test our ability to adapt or if we are being readied for a specific mission? Time will tell..."[16]

From Shephard's diary entries: "The rumors have been flying since our indoor combat training began. Most of my peers are convinced that we are being primed for a mission. No one can agree on what the mission is. I have heard the name Black Mesa Facility thrown around a lot, but I have no information about the place. The rumors are that some top-secret research is going on there. Doesn't sound too exciting to me..."[16]

From Shephard's diary entries: "The rumor has been confirmed. We are being trained for a mission at the Black Mesa Facility. All I know is that the place is being used by scientists who are doing some kind of new research. I can't imagine what we would be needed for. We were told today to be ready in case it happens tomorrow. I don't know what "it" is, but the whole thing is a little strange. I kind of hope it doesn't happen; the mission doesn't seem to have much excitement potential. I'd rather hold out for something with more likelihood of combat."[16]

The untested AI of GLaDOS is activated for the first time as one of the planned activities on Aperture's first annual bring-your-daughter-to-work day. Upon being activated, she almost instantly becomes self-aware, takes control of the Enrichment Center, locks everyone inside, and floods the Enrichment Center with a deadly neurotoxin, but is halted when she is quickly fitted with a Morality Core. She then begins a permanent cycle of testing, aimed at beating Black Mesa in the race to develop functioning portal technology.[5][6]

Barney Calhoun is assigned to a 09:00 - 22:00 Blue Shift assignment until August 15 and reports at Area 3 Medium Security Facilities at 09:00. Other unknown security guards are assigned to Red, Orange, Yellow, Green and Indigo Shifts in specific areas of Sector A, B, and C. For that timespan, the Violet Shift is on standby.[18]

The latest date as to when Gordon would begin his employment at Black Mesa.[19]Half-Life 2 and its episodes implied that Gordon was already working for a time at Black Mesa before the incident: Barney tells Gordon he owes him a beer and reminds him who he is when first meeting in the City 17 Trainstation, and Alyx tells in the City 17 Underground that Gordon and Barney would compete in the air ducts to get into Kleiner's office whenever he got himself locked out. Dr. Breen describes Gordon's time at Black Mesa as being a "brief tenure."

Gordon Freeman is being instructed by Gina Cross' hologram counterpart for an HEV Suit Training in Sector A's Training Facility. Prior to beginning the training, the scientists monitoring Gordon mention that the sensors in their equipment are not working.[19][21]

Barney Calhoun starts his tram ride from the Area 8 Topside Dormitories at Black Mesa,[20] to start his second Blue Shift day at Area 3 Medium Security Facilities in Sector C.[18] He arrives there some time before 9:00.

Otis Laurey was to be instructed by the Miller's holographic instructor counterpart for a Security Guard Training at 16:30, but he was apparently unable to attend and Cross suggests to Green to ask Barney to replace him.[19] This never happened due to the incident.

Early in the morning, Gina Cross and Colette Green are able to perform a resonance reversal. Their fate, along with Keller's, are unknown.[22]

Adrian Shephard and his squad are attacked by Alien Crafts and their Osprey crashes. As Gordon Freeman fights his way across the surface towards the Lambda Complex, Shephard is left behind (this matches the message addressed to Cooper). Race X creatures appear sometime later.

During this period of around 20 years Gordon Freeman is in stasis.[23] It starts at the aftermath of the Black Mesa Incident in May 200- and ends at Gordon's arrival in City 17 in 202-.

Portal Storms rage on Earth while Xen creatures continue to be teleported. The governments and United Nations provide the protection centers located in major cities worldwide.

The Aperture Science Enrichment Center remains in lockdown, with employees still trapped inside. The number of Aperture Science employees also likely diminishes, until there are only a few of them left.

The Combine launches an assault on Earth. Earth governments and the United Nations enter into war against them. The Seven Hour War occurs. Earth surrenders to the Combine after seven hours, and Wallace Breen, former Black Mesa Administrator, is appointed Earth Administrator.

This is the period during which Portal is set. It occurs a short amount of time after the Combine invasion of Earth,[24][25] somewhere around the year 2010.[26]

In the Aperture Science computer-aided Enrichment Center, Chell is awoken by GLaDOS who makes her perform seemingly routine tests. However Chell soon learns that the Aperture employees are long dead or escaped, and that GLaDOS is seemingly the only being left in the decaying facility. After much promise of cake at the test's conclusion, Chell is met with an incinerator, which she narrowly escapes, and works her way through the maintenance areas, despite GLaDOS's protests. Eventually, Chell finds GLaDOS's control room, and manages to escape death by neurotoxin by detaching GLaDOS's Personality Cores, and partially destroying her as a result. Chell is forced to the surface by GLaDOS's explosion, only to be dragged away from freedom and back into the facility by the Party Escort Bot. Unknown to Chell however, GLaDOS is still alive.

Doug Rattmann, being the one of the last Aperture employees alive, witnesses GLaDOS' destruction. After the Party Escort Bot drags Chell back into the facility, Rattmann feels guilty for Chell's situation and reenters the facility to help her.

At an unknown time before Gordon Freeman's awakening, Isaac Kleiner and Eli Vance each build a teleport in their respective labs a not needing a Xen relay with the aid of Alyx Vance and Judith Mossman, among others. The teleport was tested on a cat, whose fate was never revealed. They then continue working on it until Freeman's arrival.

Around 20 years after the Black Mesa Incident,[23] Gordon Freeman is awakened by the G-Man after a long period of stasis. He explores City 17, encounters the Resistance and some of his former Black Mesa colleagues. Soon after he joins their cause in the war against the Combine and triggers the uprising of City 17 and his surroundings, becoming the primary target to the Combine. After destroying the Citadel's dark fusion reactor and defeating Breen, Gordon is called into stasis by the G-Man a second time.

GLaDOS' partial destruction is followed by a period of inactivity within the Enrichment Center, during which time maintenance systems and Personality Cores maintain its functions. The facility remains in disarray, having become overgrown and dilapidated.[11] Pre-recorded 'Emergency Test Protocols' can oversee test chambers in times of cataclysmic system failure, "remain[ing] functional in apocalyptic, low power environments of as few as 1.1 volts". Chell spends this time in stasis until the events of Portal 2. At the beginning of her stasis, Chell is under supervision by a remaining Aperture employee, Doug Rattmann.

The span of time between Portal and Portal 2 is not entirely clear. During Portal 2's opening sequence, an automated 'Courtesy Call' implies that Test Subjects in stasis are revived every 50 days. However, when the player is next woken, an autonomous P.A. system reports that the time elapsed is "99999...99...". It is unknown what unit of time it refers to. Of course, it may be that the system has completely broken down and is no longer capable of accurately tracking time.

After over 50,000[26] years in stasis, Chell is awoken by Wheatley, a personality core. Wheatley insists he can secure an escape route out of the Aperture Laboratories, which instead results in the inadvertent reactivation of GLaDOS.[11] As the story progresses, Chell finds herself traversing old condemned Enrichment Spheres originating from earlier than June 15, 1961, at which time Aperture Laboratories was under the direction of Cave Johnson.